Rickettsiae are very small parasitic microorganisms (approximately 0.2 micron) which are of the taxonomical order Rickettsiales, family Rickettsia. Rickettsial diseases caused by these parasites have been very significant throughout history to both humans and animals. Human deaths caused by outbreaks of epidemic typhus and scrub typhus number in the millions. Epidemic typhus is caused by the rickettsia Rickettsia prowazeki. Scrub typhus is caused by the rickettsia Rickettsia tsutsugamushi which is still endemic in many rural areas of Southeast Asia and Japan. Rocky Mountain spotted fever, caused by Rickettsia rickettsii, is widespread in the eastern United States and is a risk in many other parts of the country.
Animal diseases caused by rickettsiae include Rocky Mountain spotted fever and canine ehrlichiosis, caused by Ehrlichia canis, both of which afflict dogs. Rickettsial diseases of horses include equine ehrlichiosis, caused by Ehrlichia equis, and Potomac fever, caused by Ehrlichia risticii. Serious losses occur to cattle from the rickettsia Anaplasma marginale. Some animal rickettsial diseases are communicable to humans, for example, Q-fever, canine ehrlichiosis and Potomac fever. Despite the widespread significance of rickettsial diseases, little has been known about the molecular biology of the rickettsiae.
Anaplasmosis is an arthropod borne hemoparasitic disease of cattle and other ruminants caused by Anaplasma marginale. Anaplasmosis occurs worldwide and severely constrains livestock production in tropical and subtropical regions. This rickettsia is transmitted by ticks, biting flies, and blood contaminated fomites to susceptible animals, where it infects red blood cells (erythrocytes). Anaplasma marginale occurs in the red blood cells as an intraerythrocytic initial body, which is a single Anaplasma marginale organism in a mature infective stage of the microbe's life cycle. The infective initial bodies reproduce by binary fission within the erythrocytes to form two to eight initial bodies which are subsequently released to infect additional erythrocytes.
During acute infection the level of these parasites increases geometrically and severe extravascular anemia occurs. Marked weight loss, abortion, and death can occur during the acute crisis caused by this parasitic infection and the resultant parasitemia. Animals that recover from the acute infection remain persistently infected and are a reservoir for transmission to susceptible animals.
Current immunoprophylaxis for anaplasmosis includes premunization with a less virulent Anaplasma marginale isolate or Anaplasma centrale, a less virulent anaplasma species. Premunization is typically followed by tetracycline treatment to control severe infection in some animals. Another immunoprophylatic approach is vaccination with a vaccine containing killed whole Anaplasma marginale organisms and host erythrocyte stroma. Premunition is successful in controlling severe clinical disease when cattle are challenged with a virulent isolate. However, clinical disease including weight loss, abortion and occasionally death may result from premunizing inoculum. This inoculum may also transmit other hemoparasites, such as Babesia, Theileria, and Trypanosoma, and viruses, such as leukemia virus, to the animal being treated. Challenge of cattle immunized with the killed Anaplasma marginale-erythrocyte stroma vaccine results in mild clinical disease and persistent infection. In addition, the presence of erythrocyte stroma in the vaccine has been shown to induce anti-erythrocyte antibodies which can be transferred through a cow's colostrum to a nursing calf thus causing the autoimmune disease neonatal isoerythrolysis.
Accordingly, there remains a strong need for improved immunization techniques effective against these and other rickettsial diseases. There also remains a continuing need for relatively simple diagnostic tests for detecting carriers of rickettsial parasites.